In turbojets having a large dilution ratio, the radius of the primary flow stream decreases from upstream to downstream in the low pressure compressor. This stream is very highly conical in the last stages of the compressor. The blades of these stages extend obliquely into the stream relative to a plane perpendicular to the axis of rotation of the compressor, i.e. obliquely relative to the radial direction of centrifugal forces.
The invention relates more precisely to bladed disks of this type in which the blades are held by respective fasteners of hammerhead type received in a peripheral groove of the disk, the groove being defined by an upstream lip and a downstream lip having surfaces connected to the bottom of the groove that form bearing surfaces against which the flanks of blade roots come to bear while the turbomachine is in operation, these bearing surfaces withstanding reaction forces with a resultant that is preferably in the plane of the centrifugal forces to which the blades are subjected.
To achieve this result, EP 0 695 856 proposes an asymmetrical hammerhead fastener, i.e. one in which the angle of the bearing surface of the upstream lip, which is the lip of larger diameter, relative to a plane perpendicular to the axis of rotation is greater than the angle formed between the bearing surface of the downstream lip and said plane. FIG. 4B of that document shows the blade-disk connection for the case in which the blade, on being subjected to a high level of axial stress, e.g. following an impact from debris ingested by the turbomachine, tends to pivot about a center of rotation C that is situated at the upstream end of the bearing surface of the downstream lip. Because of the shape of the groove and of the root of the blade, the blade can escape in the event of a major impact.
U.S. Pat. No. 5,271,718 describes blades of the symmetrical hammerhead fastener type which present platforms having ribs on their radially-inner faces that extend circumferentially and axially and that are designed to avoid vibratory resonance, two of the circumferential ribs co-operating with rings formed at the periphery of the disk to provide leaktightness in these zones. The axial thickness of the ribs is substantially equal to the axial thickness of the rings.
That document shows that the axial ribs formed on the radially-inner faces of the platforms are of height that is smaller than that of the ribs co-operating with the rings. In the event of a high level of axial stress, the ribs situated downstream supports a major fraction of the forces that are generated and they might skid axially on the downstream ring, which can lead to the blade becoming detached.
In addition, in the event of tangential stress, the ends of said ribs can skid on the rings, and even if that does not lead to the blades becoming disengaged, it can nevertheless lead to adjacent edges of two neighboring blades overlapping.
These troubles can occur in particular in a bladed disk of the type mentioned in the introduction of the present specification, in which the blades extend into a stream that is highly conical.